The Drosophila sex-determination gene fruitless is a neurally expressed gene necessary for the establishment of male sex behavior and a male-specific abdominal muscle, the muscle of Lawrence (MOL). Therefore, its study may have general application to the understanding of the genetic processes that underlie sexual dimorphism in behavior and muscle development. The extent of evolutionary conservation of this gene is, however, unknown. Evolution of this gene will thus be studied in an approach that combines genetics, molecular biology, and behavior. This will be accomplished in a number of experimental approaches. (a) Sequences encoding two of the FRU protein?s functional domains, the BTB protein dimerization domain and the 101-amino-acid male specific end, will be obtained and compared for the Drosophila species D. affinis, D. azteca, D. subobscura and D. virilis. Correlations will be drawn between fru gene sequence evolution and development of the MOL. (b) Determinations of FRU expression patterns will be made in the species by immunostaining, and then compared with the published pattern in D. melanogaster. (c) fru-related sex behavior will be determined among the species and correlated with the above fru gene structure and expression analysis. (d) Interspecific hybrids will be generated and analyzed likewise to address further evolutionary aspects of fru. (e) A mutagenesis screen will be carried out seeking genes of the MOL developmental pathway; this will employ a unique strategy independent of courtship behavior selection.